Antigenic variation in African trypanosomes by gene replacement or activation of alternate telomeres.

We have analyzed antigenic variants with a known lineage and show that there are several telomeres on which variant surface glycoprotein (VSG) genes can be expressed. These telomeres have similar restriction maps 5' to the barren region. In addition, the same VSG gene was expressed on different telomeres. Some antigenic switches in the lineage were accomplished by duplicative replacement of one VSG gene with another. Other switches occurred without duplication by transcriptional activation of an alternate telomeric VSG gene. We call the latter process telomeric activation and propose that these two processes can occur independently. We further propose that antigenic switching by telomeric activation is mediated by the regulatory system that controls which telomere is transcriptionally active, while the duplicative mechanism does not.